Spraying equipment for the electrostatic coating of surfaces is known. With this equipment, the powdered coating material, flowing pneumatically through a special channel comprising an insulator which tends to develop triboelectric charges, is charged electrically through friction effects and atomized to a highly dispersed powder cloud in the outlet opening of the channel by flow guiding elements (baffles) or radial air jets (West German (FRG) patents Nos. 1,577,757, 2,203,351 and 2,257,316).
It is a disadvantage of the known apparatus that only such powder can be used which tends to develop a very high triboelectric charge. Moreover, it operates with a relatively low powder concentration in the carrier gas, to achieve a powder charge, which is adequate for the process, by frequent contact of the powder particles with the interior wall of the flow channel.
Various measures have been proposed to increase the electrical charge on the powder particles, which are directed toward intensifying the turbulence of the flow and thus the intensity and frequency of wall contacts by the powder particles, such as the use of surface profiling in the channel wall (FRG patent No. 2,209,231), the design of the flow channel in the shape of a longer, curved charging tube of small cross sectional area (FRG patent No. 3,100,002) and the use of a friction cone (German Democratic Republic (GDR) patent No. 134,841) or a flow guide element with spiral grooves or channels (FRG utility patent No. 8,516,746) in the insulating material channel (GDR patent No. 134,841) or of special turbulence-producing equipment in the form of vanes (FRG patent No. 2,938,606), propellers (U.S. Pat. No. 3,905,330) or a fan wheel (FRG patent No. 2,451,514). Moreover, structural embodiments of the triboelectric charging channel are known, for which a reduced pressure is produced in the powder inlet section or in the outlet opening by increasing the flow velocity at the curved surfaces of insulating material by means of addition air jets. This reduced pressure leads the powder particles to the surface of insulating material (FRG patent No. 2,713,697) or, by generating a helical particle path (FRG patent No. 2,756,009), causes a more intensive contact with the wall.
Moreover, solutions for electrostatic spray equipment are known, for which the powder, by superimposing triboelectric effects, is charged by ionization processes initiated by these effects (GDR patents Nos. 106,308 and 232,595). In the inlet zone of the triboelectric charging channel, this spraying equipment contains a grounded electrode, which functions as a passive electrostatic induction ionizer. The powder particles, dispersed in carrier gas, first of all receive a charge due to triboelectric effects on contacting the wall surfaces of the channel of insulating material. While the charge on the powder particles is continuously transported out of the channel of insulating material with the flow, a charge of equal magnitude and opposite polarity remains on the wall of this channel. This charge grows constantly and induces a charge of the same polarity as the powder charge, until the field, developing between the two charges, exceeds the electrical strength of the carrier gas and gas ionization commences before the electrode. The gas ions, moving towards the wall of the channel of insulating material, have the same polarity as the powder charge produced triboelectrically. They lead to a compensation of the surface charge on the wall of the channel of insulating material and thus regenerate the surface for a further triboelectric charge. At the same time, a portion of the ions is deposited on the powder particles flowing by. The charge on the particles, produced triboelectrically, is thus increased. Alternatively, the neutral powder particles, which flow through the channel without contacting the wall, are also charged electrically.
It is a disadvantage of these solutions that the ionization processes take place in a narrow flow channel, which generally is constructed as a narrow annular gap. The surfaces of insulating material of this gap have a shielding effect with respect to the axial electrical field, so that the effective area of the ionizer electrode is limited to the initial section of the channel of insulating material. The high surface charge densities, accumulating in remote regions, can lead to interfering, spark-like sliding discharges along the interior surface of the channel of insulating material or even to dielectric breakdowns of the channel wall.